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MICROWAVE bandpass filters with compact size are always in demand for low-profile and lightweight systems. Especially in wireless communication systems, the area of RFIC system chip and the cost will be greatly reduced if bandpass filter can be integrated with active components. Many studies on reducing the large size of the conventional filters have been made.

One way to realize the on-chip MMIC filter has been proposed in [1], which utilized diagonally short-ended coupled-line and lumped capacitors. However, the realization of extremely miniaturized size is somewhat at the cost of degrading the insertion loss performance.

Recently, direct conversion architecture has been immerged for the 5GHz wireless LAN system with the potential benefit of low cost and low power [2]. In this application, bandpass filter can be only used in the front end of the system, which inquires very low insertion loss. Therefore, it is of importance to study the factors that related to the insertion loss of this new filter.

In this paper, a rigorous theoretical deduction based on the size-reduction method will be introduced to investigate the insertion loss performance. And the key physical parameter, which determines the filter attenuation, will be given. As a result, less design iterations and better filter characteristic will be achieved. The analytical model and design equations have been validated with a lot of simulations. In addition, a bandpass filter worked at 5.5GHz was designed and fabricated, whose measured result is in agreement with the predicted one.

II. ANALYTICAL MODEL DESCRIPTION

As is well known, a quarter-wave transmission line can be miniaturized to a short line with electrical length! , using the size-reduction method proposed by Hirota that utilizes combinations of short high impedance transmission line and shunt lumped capacitors [3], as shown in Fig (1). The related equations are as follows: